Signal transduction mechanisms in mesenchymal cells

Gene expression profiling of cells involved in periodontal regeneration

Understanding the molecular mechanisms involved in periodontal regeneration is important for the development of more predictable clinical techniques. This study aimed to identify these mechanisms by comparing the gene expression profiles of cells derived from regenerating defects with patient-matched periodontal ligament cells. Gene profiling was carried out via Affymetrix U133A arrays containing probes for 22,000 genes. Robust differences in gene expression were obtained by identifying genes that consistently changed by a minimum of 2-fold. Analysis of molecular function as designated by gene ontology (GO) identified differentially regulated mechanisms including protein metabolism, tyrosine kinase activity, and skeletal development. The differentially expressed genes could be broadly divided into the categories of protein biosynthesis and turnover, structural constituents of the cytoskeleton and extracellular matrix, and signal transduction. The differential expression of 4 genes (EGR-1, elastin, osteoprotegerin, and IGFBP3) was confirmed via real-time polymerase chain reaction (PCR). Further, the expression of another 2 differentially expressed transcripts, decorin and biglycan, was immunohistochemically confirmed in a periodontal wound healing model and the protein expression was consistent with the pattern of gene expression. This study gives insight into the molecular processes involved in periodontal regeneration and identifies cell markers that are characteristic of regenerating periodontal tissues.

Effect of mechanical loading on periodontal cells

Mechanical loading is an important regulatory factor in alveolar bone homeostasis, and plays an essential role in maintaining the structure and mass of the alveolar processes throughout lifetime. A better understanding of the cellular and molecular responses of periodontal cells is a prerequisite for further improvements of therapeutic approaches in orthodontics, periodontal and alveolar bone repair and regeneration, implantology, and post-surgical wound healing. The purpose of this review is to provide an insight into some cell culture and animal models used for studying the effects of mechanical loading on periodontal cells, and into the recent developments and utilization of new in vivo animal models. There has been an increased awareness about the need for improvement and development of in vivo models to supplement the widely used cell culture models, and for biological validation of in vitro results, especially in the light of evidence that developmental models may not always reflect bone homeostasis in an adult organism. Due to the limitations of in vivo models, previous studies on mechanical regulation of alveolar bone osteoblasts and cementoblasts mostly focused on proliferative responses, rather than on the stimulation of cell differentiation. To address this problem, we have recently characterized and implemented a mouse osteoinductive tooth movement model for studying mechanically induced regulation of osteoblast- and cementoblast-associated genes. In this model, a defined and reproducible mechanical osteogenic loading is applied during a time course of up to two weeks. Regulation of gene expression in either wild-type or transgenic animals is assessed by a relative quantitative measurement of the level of target mRNAs directly within the subpopulations of periodontal cells. To date, results demonstrate a defined temporal pattern of cell-specific gene regulation in periodontal osteoblasts mechanically stimulated to differentiate and deposit bone matrix. The responses of osteoblast-associated genes to mechanical loading were 10- to 20-fold greater than the increase in the numbers of these cells, indicating that the induction of differentiation and an increase of cell function are the primary responses to osteogenic loading. The progression of the osteoblast phenotype in the intact mouse periodontium was several-fold faster compared with that in cultured cells, suggesting that the mechanical signal may be targeting osteoblast precursors in the state of readiness to respond to an environmental challenge, without the initial proliferative response. An early response of alkaline phosphatase and bone sialoprotein genes was detected after 24 hrs of treatment, followed by a concomitant stimulation of osteocalcin and collagen I between 24 and 48 hrs, and deposition of osteoid after 72 hrs. Although cementoblasts constitutively express biochemical markers similar to those of osteoblasts, distinct responses of osteocalcin, collagen I, and bone sialoprotein genes to mechanical loading were observed in the two cell phenotypes. This finding indicates that differential genetic responses to mechanical loading provide functional markers for distinction of the cementoblast and osteoblast phenotypes.

Osteoblasts induce osteopontin expression in response to attachment on fibronectin: demonstration of a common role for integrin receptors in the signal transduction processes of cell attachment and mechanical stimulation

Osteopontin is a predominant integrin binding protein of bone and its expression has been shown to be induced by mechanical stimuli within osteoblasts (Toma et al. [1997] J. Bone Miner. Res. 12:1626-1636). The present studies examined if the cell adhesion would mimic the mechano-transduction that stimulated opn mRNA expression and whether integrin receptors were involved in these processes. Osteopontin mRNA expression was induced three- to four-fold, 24 hours after embryonic chicken calvaria osteoblast attachment to fibronectin (FN), however no induction was observed if the cells were plated on tissue culture plastic alone. Osteopontin mRNA induction in response to cell attachment on FN was dependent on new protein synthesis and the activation of a tyrosine protein kinase(s) but unlike mechano-induction was independent of the maintenance of the cell's microfilament structure. Integrin receptor(s) were shown to be involved in mediating the signal transduction processes of both cell attachment and mechanical stimulation since incubation of osteoblasts with the integrin binding peptide RGDS partially blocked the induction of opn expression in response to both stimuli. Interestingly, incubation of the osteoblasts that were adherent on tissue culture plastic alone with the RGDS peptide lead to an induction in opn expression suggesting that integrin occupancy by itself was sufficient to initiate the signal transduction process that induced opn expression. In order to assess the role of integrin occupancy vs. focal adhesion complex formation that accompanies cell attachment, in the signal transduction process that induces opn expression, receptor clustering was stimulated pharmacologically with bombesin or lysophosphatidic acid in osteoblasts attached to tissue culture plastic. Neither compound in the absence of occupancy of the integrin receptors was capable of stimulating opn expression in attached cells, however if the cells were placed in suspension pharmacological mediation of receptor clustering and integrin occupancy were additive in their effect of inducing opn expression. These data demonstrate that induction of opn expression by mechanical stimuli and cell attachment are commonly mediated through integrin receptor(s). However, when cells are attached receptor clustering alone which accompanies focal adhesion formation was incapable of mediating signal transduction suggesting that receptor occupancy was a prerequisite to the signal transduction process that leads to the induction of opn mRNA expression.

Short-chain carboxylic-acid-stimulated, PMN-mediated gingival inflammation

This communication reviews the effects of short-chain carboxylic acids on human cells of importance to the periodontium. The central hypothesis is that these acids can alter both cell function and gene expression, and thus contribute to the initiation and prolongation of gingival inflammation. Short-chain carboxylic acids [CH3-(CH2)x-COOH, x < 3] are metabolic intermediates with a broad range of apparently paradoxical biological effects. For example, lactic acid (CH3-CHOH-COOH), a 3-carbon alpha-hydroxy-substituted acid, is widely recognized for its cariogenicity. Lactic acid, however, also occurs in tropical fruits, and is the active ingredient in a variety of anti-wrinkle creams developed by dermatologists. In marked contrast, the unsubstituted 3-carbon propionic acid (CH3-CH2-COOH) is used as a food preservative and is the active principle for one class of non-steroidal anti-inflammatory agents. Interestingly, the addition of one carbon to propionic acid dramatically changes the biological effects. The unsubstituted 4-carbon butyric acid (CH3-CH2-CH2-COOH) is used by hematologists as a de-differentiating agent for the treatment of sickle cell anemia, but by oncologists as a differentiating agent for cancer chemotherapy. Finally, acting either individually or in concert, these acids can increase vascular dilation. Clearly, these acids, while metabolically derived, have a number of very divergent activities which are cell-type-specific (Fig. 1). It may be telling that periodontal bacteria produce these acids in millimolar concentrations, and that these bacteria can be characterized by their acid production profiles. It is no less interesting that these acids occur in the gingival crevices of human subjects with severe periodontal disease at millimolar levels which are > 10-fold higher than those found in mildly diseased subjects, and are undetectable in healthy subjects. Further, when applied directly to healthy human gingiva, short-chain carboxylic acids stimulate a gingival inflammatory response and inflammatory cytokine release. At the cellular level, these acids inhibit proliferation of gingival epithelial and endothelial cells, and inhibit leukocyte apoptosis and function, but can stimulate leukocyte cytokine release. At the molecular level, these acids can stimulate neutrophil gene transcription, translation, and protein expression. Thus, the likelihood is high that these acids, in addition to their cariogenic activity, can promote and prolong gingival inflammation. Our challenge will be to identify the cell or cells of the periodontium which respond to short-chain carboxylic acids, to delineate their responses and the molecular mechanism(s) of these effects, and to categorize the aspects of the inflammatory components which damage and those which protect the host. With this information, it may be possible to begin to rationally identify and test pharmaceutical agents which diminish the harmful aspects, while enhancing the beneficial components, of the inflammatory response.

Integrated processes responsible for soft tissue healing

Wounded soft tissues undergo repair through a complex series of interrelated events that involve both physical and chemical activities. These processes are currently undergoing extensive investigation as efforts are directed toward achieving augmented and accelerated healing. Early wound-healing research focused on expanding traditional histologic descriptions of tissue healing by attempting to characterize the environment and biologic mediators responsible for healing. These initial studies successfully identified a number of agents and physiochemical factors present in healing wounds, but their precise roles and importance remain largely unknown. This review article summarizes the current literature on soft tissue healing. An effort has been made to correlate the activities of the major growth factors and cytokines with the individual reparative processes including the inflammatory response, hemostasis, fibroplasia, angiogenesis, and remodeling. Explanations and characteristics of growth factor function as well as brief descriptions of several major factors and their spectrum of activity are also provided.